When two-piece aluminum draw and iron (D&I) beverage cans were first made in the mid-1960's, the cans were quite different from today's cans. Not only were the cans 70% heavier, the shape was also different. Since the aluminum can was competing against the three-piece steel can which it would eventually supplant, it necessarily had the same shape. The size of the 12-ounce beverage can in the mid-1960's was 211.times.413. Therefore, the can body was not necked prior to a flanging operation in which an outwardly extending peripheral flange was formed at one end of the can body to receive, and be seamed to, a can end after filling with beverage.
The 211 diameter configuration (can-maker's terminology referring to a diameter of 2 11/16") caused two major problems in the two-piece aluminum D&I can. The first problem was split flanges. Specifically, in the flanging operation, the metal was expanded from the 2.6" body diameter to a 2.8" flange diameter, i.e., a 7.7% increase. This obviously create circumferential tension in the flange which resulted in a tendency for it to split. Split flanges resulted in leakage from the can seams which was a major problem. The second problem related to conveying the flanged cans. When adjacent cans were allowed to touch, flange damage would occur and conveying jams were frequent because of the way the cans would tilt when in flange-to-flange contact which created clearance between the can bodies.
Although many improvements were made to lessen the adverse impacts of the foregoing problems, the solution which emerged in the mid-1960's was the necking process Necking reduced the diameter of the open end of the can prior to flanging which allowed a smaller end (e.g., a 209 end which is 2 9/16" diameter in can-maker's terminology) to be used. The resulting configuration greatly reduced the tendency for split flanges since the flange diameter in the necked can is only 2.3% greater than the body diameter. Necking also made conveying the cans easier since, with only slight flange overlap, the cans would contact body-to-body. Seamed 209 cans could contact body-to-body without tilting.
The necking process was instrumental in the subsequent success of the two-piece D&I beverage can. In the decade following the introduction of the 209 necked can, the three-piece steel can virtually disappeared from the can beverage market.
In the late 1970's, the necking process was revisited as a means of achieving further lightweighting and reduced costs. If the cans were necked to a smaller diameter, then a smaller, lighter, less expensive can end could be used. During the following years, the industry moved from the 209 neck to a 206 neck. By the mid-1980's, most commercial can-makers considered the 206 can to be industry standard.
Three different necking processes were used to produce the 206 aluminum can. In one process, a four-stage die necking procedure resulted in each successively formed neck reducing the diameter by about 0.085". In this process, four distinct necks are formed on the can. This process is called "quad-neck." Another process is a six-stage die necking process whereby each step reduces the diameter about 0.055" and the necks blend together in a continuous profile. This process is called "smooth die neck." The third type of necking process is a combination of either two or three die necks followed by a spin necking operation. Each of the die necking operations reduces the diameter by about 0.075-0.110" and the spin necking operation reduces it by 0.110". The spin necking process smooths all but the first die neck which leaves one obvious neck that blends into a continuous profile. This process is called "spin necking."
A renewed interest in cost competitiveness has resulted in the production of even smaller diameter can ends. As can-makers ponder the possibility of a 204 can end and smaller necks, they necessarily revisited the can design criteria. First and foremost, the capacity of the can must be maintained without changing the can height or diameter. This means that as the neck diameter decreases, the neck angle would ideally become greater so as to maintain the neck shoulder location and not encroach upon the volume of the can. A side benefit of a steeper neck angle is reduced metal usage. Can-makers typically employed thicker metal in the neck area of the can to facilitate necking and flanging. Therefore, a steeper, shorter neck means reduced length for the thicker metal which results in the reduced metal usage. A third advantage of a steeper neck is increased billboard, i.e., the cylindrical portion of the can available for customer graphics.
An additional consideration in the selection of a necking process is the diameter reduction capability for each step. The greater the reduction, the fewer steps are needed, thereby reducing costs and streamlining the process. Aesthetics is also a consideration. Finally, ease of manufacturing is a factor which must be considered in selecting a necking process. Any other advantages can be lost if productivity in the necking tooling is diminished because of a more critical necking process.
The foregoing considerations led to the development of a process now known in the industry as "spin flow necking." A particularly promising spin flow process and apparatus are disclosed in U.S. Pat. No. 4,781,047, issued Nov. 1, 1988, to Bressan et al, which is assigned to Ball Corporation and is exclusively licensed to the assignee of the present application, Reynolds Metals Company. The disclosure of this patent is hereby incorporated by reference herein in its entirety. It concerns a process where an externally located free spinning forming roll 11 is moved inward and axially against the outside wall C' of the open end C" of a rotating trimmed can C to form a conical neck at the open end thereof. With reference to FIG. 1, a spring-loaded holder or slide roll 19 supports the interior wall of the can C and moves axially under the forming force of the free roll 11. This is a single operation where the can rotates and the free roll 11 rotates so that a smooth conical necked end is produced. In practice, the can is then flanged. The term "spin flow necking" is used in this application to refer to such processes and apparatus, the essential difference between spin flow necking and other types of spin necking being the axial movement of both the external roll 11 and the internal support 19.
More specifically, the spin flow tooling assembly 10 depicted in FIG. 1 (corresponding to FIG. 1 of the Bressan et al '047 patent, supra) includes a necking spindle shaft 16a rotatable about its axis of the rotation A by means of a spindle gear 16 mounted to the shaft between front and rear bearings (not shown). The slide roll 19 is mounted to the front end of the necking spindle shaft 16a through a slide mechanism 28, keyed to the shaft, which permits co-rotation of the roll 19 while allowing it to be slid by the necking forces described more fully below in the axially rearward direction B' away from the eccentric freewheeling roll 24 located adjacent the front face of the slide roll. The axially fixed idler roll 24, having an axis of rotation B which is parallel to and rotatable about spindle axis A, is mounted via bearings 16b and 23 to an eccentrically formed front end of an eccentric roll support shaft 18. This shaft 18 extends through the necking spindle shaft 16a. The spindle shaft 16 is rotated by the spindle gear 16 without rotating the eccentric roll support shaft 18.
The outer forming roll 11 is mounted radially outwardly adjacent the slide and eccentric rolls 19,24.
The container slide roll 19 is shaped with a conical leading edge 19a designed to first engage the open end C" of the container C to support same for rotation about spindle axis A under the driving action of the necking spindle gear 16 which may be driven by the same drive mechanism driving each base pad assembly 29 engaging the container bottom wall. Slide roll 19 is also free to slide axially but is resiliently biased into the container open end C" via springs 20 which may be of the compression type.
In operation, the container open end C" engages and is rotated by the slide roll 19. The eccentric roll 24 is then rotated into engagement with a part of the inside surface of the container side wall C' located inwardly adjacent the open end C". With reference to FIGS. 2A-2E, the external forming roll 11 then begins to move radially inward into contact with the container side wall C' spanning the gap respectively formed between the conical faces 19a,24e of the slide and eccentric rolls 19,24. More specifically, the side wall C' of the spinning container body C is initially a straight cylindrical section of generally uniform diameter and thickness which may extend from a pre-neck (not shown) previously formed in the container side wall such as by static die necking. As the external forming roll 11 engages the container side wall C', it commences to penetrate the gap between the fixed internal eccentric roll 24 and the axially movable slide roll 19, forming a truncated cone (FIG. 2B). The side wall of the cone increases in length as does the height of the cone as the external forming roll chamfer 11c continues to squeeze or press the container metal along the complemental slope or truncated cone 24e of the eccentric roll 24 as depicted in FIG. 2C. The cone continues to be generated as the external forming roll 11 advances radially inwardly (the slide roll 19 continues to retract axially as a result of direct pushing contact from roll 11 through the metal) until a reduced diameter 124 is achieved as depicted in FIGS. 2C and 2D. As the cone is being formed, the necked-in portion 124 or throat of the container C conforms to the shape of the forming portion of the forming roll 11. The rim portions 123 of the neck which extend radially outwardly from the necked-in portion 124 are being formed by the complemental tapers 11b,19a of the forming roll 11 and the slide roll 19 to complete the necked-in portion.
A plurality of spin flow necking tooling assemblies embodying the above-identified tooling, or the improvements according to the present invention described hereinbelow, may be incorporated in a multi-station spin flow necking machine of a type disclosed in patent application Ser. No. 929,932 being filed concurrently herewith and commonly assigned, entitled "Spin Flow Necking Apparatus and Method of Handling Cans Therein" incorporated by reference herein it its entirety.
The above-described spin flow necking process, while producing a large diameter reduction in the open end of the container C (e.g., 0.350"), has various drawbacks when applied to two-piece aluminum can manufacture. One drawback, for example, is grooving of the neck at the initial point of contact between rolls 11,19 in FIG. 2B which occurs on the inside of the container as a result of the small radii on the forming roll pushing past and against the small radii on the slide roll as the forming roll moves radially inwardly and axially rearwardly during the necking process along the chamfer 24e of the eccentric roll. Due to the spring force 20 urging the slide roll 19 toward the eccentric roll 24, the metal caught between these colliding radii which are forcefully pressed together under spring bias, actually results in the grooving phenomenon on both the inner and outer surfaces of the neck. On the inside surface, this grooving results in metal exposure (i.e., wearing away of the protective coating) which often allows the beverage to "eat through" the container side wall C'. It has also been discovered that such grooving often results in actual cutting of the metal as the form roll 11 is radially inwardly advanced from the position depicted in FIG. 2B to that of FIG. 2C.
As the form roll 11 moves into its radially inwardmost position depicted in FIG. 2E, the spring pressure acting against the slide roll 19 in the direction of the forming roll disadvantageously results in pinching of the end of the flange-like portion 123 and undesirable thinning of the metal. In some cases, particularly when necking a can to smaller diameters (e.g., 204 or 202), the edge is sometimes thinned down to a knife edge.
It is accordingly an object of the present invention to prevent grooving of the container side wall or neck during the spin flow necking process.
Another object is to control the interaction of the outer form roll with the inner slide roll to ensure that the form roll acts directly on the metal at appropriate instances while preventing excessive interaction which may result in grooving.
Still a further object is to prevent excessive thinning of the flange type edge by preventing excessive force from being applied to the edge by the form and slide rolls.
Yet another object is to increase the spring force initially urging the slide roll towards the eccentric roll to allow a snug fit to occur between the container open end and the slide roll outer surface for improved support of the container open end on the slide roll during spin flow necking.